The present invention generally relates to scanning systems capable of reading bar codes. More particularly, the present invention provides low quiescent current driver circuitry for the scanning element motor.
Some scanning systems of the laser (light amplification by stimulated emission of radiation) type focus a laser beam upon a motor driven, rotating or dithering scanning mirror such that the laser beam forms a scanning pattern across a target bar code. The scanning laser beam is back-reflected to a photodetector, which determines the intensity of the back-reflected laser beam and outputs a current in proportion thereto. Thus a varying signal is output by the photodetector as the laser beam sweeps across a pattern of light and dark "bars" in a bar code.
Control circuitry controls the cooperation and coordination of the components (including the timing) and converts the photodetector output signal into useful form. Scanning speed is chiefly controlled by the speed of the motor.
The preferred embodiment of the present invention has a three-phase, direct current (DC) motor of the brushless type. Rather than using a commutator (for appropriately reversing the magnetic fields to "pull" the armature around its axis), varying signals (commutation logic) are applied to the appropriate windings to cause the magnetic fields to constantly change. The commutation logic is microprocessor controlled.
A three-phase motor is used instead of a single-phase motor since it provides for constant instantaneous power, and therefore a constant speed--an important feature for scanning operations.
Hand-held scanning systems such as the preferred embodiment have practical weight and size requirements for easy handling. Thus small components--especially the scanning mirror motor--are needed. Prior art scanning system motor driver circuits which normally drive motors with approximately 100 milliamps operating current require much more quiescent current (2 milliamps for example) than is practical for a small motor drawing, for example, 18-20 milliamps (the preferred embodiment) of operating current. Thus the driver circuitry and battery power source are much larger than desirable.